The cost increase of the next level is not the same for every investment. From the beginning of the game, generally speaking each of the properties is a reasonable investment, however since the increase in revenue is linear with level increase and cost is exponential, some become a better value per dollar later in the game. Base revenue is based off the initial return multiplied by the level. This does not include the threshold bonuses. The threshold bonuses in mid- to late-game account for the vast majority of the revenue increases.

Investment

Base Level Cost

Level Cost Increase

Gas Royalties

$5

8%

Oil Royalties

$70

8%

Gas Well

$450

11%

Oil Well

$21,000

9%

Oil Sands

$160,000

10%

Shale Play

$2.2 million

8%

Omani Field

$19.4 million

8%

Saudi Field

$620 million

7%

Reasoning

From an in game perspective, not all investments are equal, and some investments are downright bad.

If all the properties had the same price multiplier, then at all times you'd know exactly what to do.

Get all properties to 1000

Get all properties to 1100

etc.

With the imbalanced level costs, you have to think about whether getting the next achievement in one of your good properties or saving up to get the Gas Wells to the next level and helping all properties is a better deal.

This makes the completion of "Classic Mode" at 1800 Gas Wells feel more rewarding.

Near the end of "Classic Mode" it really lets some underused specialists shine.